In many situations, unwanted mechanical vibration can be transmitted from a source via pipes, beams or other flexible members. One example is the transmission of vibrations from a spacecraft or satellite to antennae or other sensitive equipment mounted on it at the end of a thin supporting beam. Typically such a beam will not be damped and the disturbing vibrations will cause disturbances at the antenna or vibration in the equipment and so interfere with its normal function.
Another example is the transmission of vibration from heavy machinery such as ships engines via pipe-work connected to it.
Various publications deal with the reduction of transverse vibrations of a beam by employing multiple actuators to introduce artificial damping, and thus to gradually remove the energy of vibration from the beam. In particular, reference is made to U.S. Pat. No. 4,817,768 to Schumacher. Such a system removes vibrational energy from a beam at a rate dependent on the energy it contains, which leads to an exponential decay in its level of vibration with time after the energy has been injected. Ifvenergy is injected continuously then a system such as that disclosed in Schumacher will only reduce the level of vibration of the beam, but cannot entirely remove it.
Various further publications deal with the cancellation of transverse modes of vibration in beams using multiple actuators.
In particular reference is made to Schaechter, D B, Eldred, D B "Experimental Demonstration of the Control of Flexible Structures", J Guidance 7 (5) 1984, pp 527-534, and Mace, B R, "Active Control of Flexural Vibrations", J Sound and Vib. 114 (2), 1987, pp 253-270.
It is an object of the present invention to prevent the transmission of low frequency vibrational energy along the beam regardless of whether it involves transverse or other types of vibration, for example torsional vibration about the beam axis or compressional vibration parallel to the axis. By the use of the invention the energy is not absorbed but is caused to return to its source, leaving the portion of the beam which it is desired to protect from vibration preferably entirely undisturbed. According to one aspect of the invention this is accomplished by a single compound actuator attached to the beam.
For the purposes of this application an `actuator` is considered to be a device capable of imposing vibrational forces on a body to which it is attached, the forces being entirely controllable by an electrical signal supplied to the actuator.
The term `low frequency` will be clarified and given a precise meaning in the section below dealing with a preferred embodiment of the invention.
Items such as beams and pipes and the like flexible members will herein also be referred to as elongate members.
Furthermore, references herein to a source of vibration are to be understood as meaning any source whether translational, torsional or compressional,